Provide all voxel iteration functions inside voxel

[kumavis]

From #9
Provide all voxel iteration functions (raycast, moore neighbourhood, etc.) inside voxel, where they can benefit from not having to look up the chunk on every iteration.

[deathcap]

and/or a generic voxel iteration method (something like voxel/voxel-clientmc#5 (comment)?), for the same reason: avoiding to calculate and lookup the chunk index and object for each voxel coordinate